Printed circuit boards having pads for solder balls and methods for the implementation thereof

ABSTRACT

A printed circuit board includes a group of pads suitable to be soldered to a respective group of solder-balls of a device. Each pad of the group has a crack initiation point on its perimeter at a location where cracks in a solder-ball are anticipated to start after that solder-ball is soldered to that pad. For a pad of that group having a microvia located therein, a center of that microvia is located farther than a center of that pad from its crack initiation point. For a pad of that group having a trace merging along a portion of its perimeter, that portion does not include a vicinity of that crack initiation point.

BACKGROUND OF THE INVENTION

A package suitable to contain one or more semiconductor dies maycomprise solder-balls in a ball grid array (BGA). Any or all of thesolder-balls may serve as external electrical terminations of thepackage. A package with one or more semiconductor dies installed thereinis known as a device.

Once a device is soldered onto a printed circuit board (PCB), mechanicaland/or thermal and/or thermomechanical strains may occur between thepackage and the PCB. In packages comprising BGA solder-balls, suchstrains may result in damage to the soldered solder-balls over time.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are illustrated by way of example and notlimitation in the figures of the accompanying drawings, in which likereference numerals indicate corresponding, analogous or similarelements, and in which:

FIGS. 1-3 show a top view of an exemplary printed circuit boardscomprising a pad suitable to be soldered to a respective solder-ball ofa device and merging with a trace according to some alternativeembodiments of the invention;

FIGS. 4-7 shows a top view of an exemplary printed circuit boardscomprising a pad suitable to be soldered to a respective solder-ball ofa device and having a microvia located therein according to somealternative embodiments of the invention;

FIG. 8 shows a top view of an exemplary printed circuit boardscomprising pad suitable to be soldered to a respective solder-balls of adevice and having microvias located therein according to someembodiments of the invention;

FIG. 9 is a top view of an exemplary printed circuit board in accordancewith some embodiments of the invention;

FIG. 10 is a top view of an exemplary printed circuit board havingdevices installed thereon, in accordance with some embodiments of theinvention; and

FIG. 11 is a top view of an exemplary apparatus including an exemplaryprinted circuit board, the exemplary printed circuit board havingdevices installed thereon, in accordance with some embodiments of theinvention.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn to scale.For example, the dimensions of some of the elements may be exaggeratedrelative to other elements for clarity.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of embodiments of theinvention. However it will be understood by those of ordinary skill inthe art that the embodiments of the invention may be practiced withoutthese specific details. In other instances, well-known methods,procedures, components and circuits have not been described in detail soas not to obscure the embodiments of the invention.

A printed circuit board (PCB) may comprise metal pads on its externalconductive layer that are soldered to respective solder-balls of adevice. Such solder balls may suffer from mechanical fatigue due to, forexample, cyclic stress, or thermo-mechanical fatigue due to, forexample, coefficient of thermal expansion (CTE) mismatch. As a result ofthat mechanical fatigue, any or all of these solder balls may developcracks (fractures), which may gradually degrade the reliability of theelectrical coupling these balls provide between the PCB and asemiconductor die contained in the device, and may result in a totalfailure of the solder joint. Consequently, the reliability of the systemin which that PCB is installed may be degraded over time, possiblyresulting in a total system failure.

Such cracks may start developing in a solder ball at a point (e.g.“crack initiation point”) that is close to the perimeter of the pad, andmay propagate through the solder ball in such a way that a projection ofthe crack onto the pad forms a substantially straight line crossing thecenter of the pad to a point on the perimeter of the pad that isopposite to the crack initiation point.

Moreover, and although the scope of the invention is not limited in thisrespect, a projection onto a PCB of the crack propagation directions insolder balls of a device soldered to pads of the PCB may point toward acommon point, and the crack initiation point of each solder ball may benear the point of the pad that is the farthest from that common point.

Using currently known and future methods, it may be possible toanticipate the location of the crack initiation point on the perimeterof the pad, and to further anticipate the crack propagation direction.

A solder-ball soldered to a solder-mask defined pad (i.e. a pad that ispartially covered with solder mask along its perimeter) may be moreprone to crack initiation than a solder-ball soldered to a metal definedpad (i.e. a pad that is not covered with solder mask). For example, thismay be due to a protruded solder mask that leads to stress concentrationand shape formation on solder-mask defined pads, possibly resulting in“necking”. In another example, this may be due to thermo-mechanicaltensions between the solder-ball and the solder-mask. Moreover, asolder-ball soldered to a pad that has a trace merging with it (i.e. thepad and the trace are part of the same metal layer of the PCB) along aportion of the pad's perimeter that includes a vicinity of the crackinitiation point, may be more prone to crack initiation than asolder-ball soldered to a pad that has a trace merging with it along aportion of the pad's perimeter that does not include a vicinity of thecrack initiation point.

A metal pad, suitable to be soldered to a respective solder ball of adevice, may have a microvia located therein. A microvia may be ametal-plated through-hole in at least the metal pad and the isolationlayer of the PCB underneath the metal pad, providing electrical couplingbetween the metal pad and one or more internal conductive layers of thePCB and/or another external conductive layer of the PCB.

During the process of soldering a pad having a microvia to a solder-ballof a device, the solder-ball may change its shape. The external shape ofthe solder-ball may change by, for example, having a part of thesolder-ball material filling, at least in part, the through-hole of themicrovia. The internal shape of the solder-ball may change, for example,by a formation of a void in the solder-ball material above the microvia.Such a void inside the soldered solder-ball may weaken the solder joint.

The inventors have come to the conclusion that the length of the pathbetween the crack initiation point and the surface of the void mayaffect the solder fatigue. A longer path may prolong the fatigue life ofa solder joint.

Although the size of a void in a solder ball may vary, a void may besubstantially centered above the center of the microvia, andconsequently, the longer the path between the crack initiation point andthe center of the microvia, the longer the path between the crackinitiation point and the surface of the void.

According to some embodiments of the invention, instructions may beapplied to the implementation of microvias inside pads suitable to besoldered to respective solder-balls of devices, and to theimplementation of merging such pads with traces. Instructions accordingto embodiments of the invention may be used in generating one or morefiles for manufacturing of a printed circuit board using. Such files maybe generated, for example, by a computer aided design (CAD) software inan automatic process or in a human-assisted process. Differentpriorities may be assigned to different instructions, and it may happenin the implementation of a particular printed circuit board that not allinstructions can be implemented for all the pads and all the traces towhich the instructions apply.

FIGS. 1-3 present three exemplary alternative implementations, usinginstructions according to some embodiments of the invention, of tracesmerged with pads that are suitable to be soldered to respectivesolder-balls of a device.

FIGS. 4-7 present four exemplary alternative implementations, usinginstructions according to some embodiments of the invention, ofmicrovias in pads that are suitable to be soldered to respectivesolder-balls of a device

FIG. 1 shows an exemplary PCB 1 comprising a pad 2 suitable to besoldered to a solder-ball of a device. Pad 2, which for the clarity ofthe explanation is substantially round, merges with a trace 3 along aportion 4 of a perimeter 5 of pad 2 according to some embodiments of theinvention. A point 6 is the anticipated location of the crack initiationpoint on the perimeter of pad 2.

A first instruction according to some embodiments of the invention usedin the implementation of pad 2 and trace 3 is to merge pad 2 and trace 3along a portion of perimeter 5 that does not include an arc 7 that is inthe vicinity of point 6.

A second instruction according to some embodiments of the invention usedin the implementation of pad 2 and trace 3 is to merge pad 2 and trace 3along a portion of perimeter 5 a length of which is no longer than onequarter of the length of perimeter 5.

FIG. 2 shows an exemplary PCB 11 comprising a pad 12 suitable to besoldered to a respective solder-ball of a device. Pad 12, which for theclarity of the explanation is substantially round, merges with a trace13 along a portion 14 of a perimeter 15 of pad 12. A point 16 is theanticipated location of the crack initiation point on the perimeter ofpad 12. A vector 19 beginning at point 16 is the projection onto pad 12of the anticipated crack propagation direction. A line 18 is a tangentto perimeter 15 at the middle point of portion 14. Line 18 issubstantially parallel to vector 19.

A first instruction, according to some embodiments of the invention,used in the implementation of pad 12 and trace 13 is to merge pad 12 andtrace 13 along a portion of perimeter 15 that does not include an arc 17that is in the vicinity of point 16.

A second instruction, according to some embodiments of the invention,used in the implementation of pad 12 and trace 13 is to merge pad 12 andtrace 13 along a portion of perimeter 15 so that a tangent to perimeter15 at a middle point of the merging portion will be substantiallyparallel to the projection onto pad 12 of the anticipated crackpropagation direction.

A third instruction, according to some embodiments of the invention,used in the implementation of pad 12 and trace 13 is to merge pad 12 andtrace 13 along a portion of perimeter 15 a length of which is no longerthan one quarter of the length of perimeter 15.

FIG. 3 shows an exemplary PCB 21 comprising a pad 22 suitable to besoldered to a respective solder-ball of a device. Pad 22, which for theclarity of the explanation is substantially round, merged with a trace23 along a portion 24 of a perimeter 25 of pad 22 according to someembodiments of the invention. A point 26 is the anticipate location ofthe crack initiation point on the perimeter of pad 22. A vector 29initiated at point 26 is the projection onto pad 22 of the anticipatedcrack propagation direction.

A first instruction, according to some embodiments of the invention,used in the implementation of pad 22 and trace 23 is to merge pad 22 andtrace 23 along a portion of perimeter 25 that does not include an arc 27that is in the vicinity of point 26.

A second instruction, according to some embodiments of the invention,used in the implementation of pad 22 and trace 23 is to merge pad 22 andtrace 23 along a portion of perimeter 25 so that a straight line joiningpoint 26 and the middle point of the merging portion will be parallel tovector 29.

A third instruction, according to some embodiments of the invention,used in the implementation of pad 22 and trace 23 is to merge pad 22 andtrace 23 along a portion of perimeter 25 a length of which is no longerthan one quarter of the length of perimeter 25.

FIG. 4 shows an exemplary PCB 31 comprising a pad 32 suitable to besoldered to a respective solder-ball of a device. Pad 32, which for theclarity of the explanation is substantially round, comprises a microvia33 according to some embodiments of the invention. A point 36 is theanticipated location of the crack initiation point on perimeter 35 ofpad 32. The center 34 of microvia 33 is farther than the center 37 ofpad 32 from point 36. For the clarity of the drawing, a dashed curve 39is shown, which is an equidistance curve from point 36 containing thecenter 37 of pad 32.

An instruction according to some embodiments of the invention, used inthe implementation of pad 32 and microvia 33 is to locate microvia 33inside pad 32 such that the center of microvia 33 is farther than thecenter of pad 32 from point 36.

FIG. 5 shows an exemplary PCB 41 comprising a pad 42 suitable to besoldered to a respective solder-ball of a device. Pad 42, which for theclarity of the explanation is substantially round, comprises a microvia43 according to some embodiments of the invention. A point 46 is theanticipated location of the crack initiation point on a perimeter 45 ofpad 42. The center 44 of microvia 43 is farther than the center 47 ofpad 42 from point 46. In addition, at least one point of perimeter 48 ofmicrovia 43 is located on perimeter 45 of pad 42. For the clarity of thedrawing, a dashed curve 49 is shown, which is an equidistance curve frompoint 46 containing the center 47 of pad 42.

A first instruction according to some embodiments of the invention, usedin the implementation of pad 42 and microvia 43 is to locate microvia 43inside pad 42 such that the center of microvia 43 is farther than thecenter of pad 42 from point 46.

A second instruction according to the invention, used in theimplementation of pad 42 and microvia 43 is to locate microvia 43 insidepad 42 such that at least one point of perimeter 48 of microvia 43 islocated on perimeter 45 of pad 42.

FIG. 6 shows an exemplary PCB 51 comprising a pad 52 suitable to besoldered to a respective solder-ball of a device. Pad 52, which for theclarity of the explanation is substantially round, comprises a microvia53 according to some embodiments of the invention. A point 56 is theanticipated location of the crack initiation point on a perimeter 55 ofpad 52. A vector 58 beginning at point 56 is the projection onto pad 52of the anticipated crack propagation direction. The center 54 ofmicrovia 53 is located on vector 58 and is farther than the center 57 ofpad 52 from point 56. For the clarity of the drawing, a dashed curve 59is shown, which is an equidistance curve from point 56 containing thecenter 57 of pad 52.

A first instruction according to some embodiments of the invention, usedin the implementation of pad 52 and microvia 53 is to locate microvia 53inside pad 52 such that the center of microvia 53 is farther than thecenter of pad 52 from point 56.

A second instruction according to some embodiments of the invention,used in the implementation of pad 52 and microvia 53 is to locate center54 of microvia 53 on vector 58.

FIG. 7 shows an exemplary PCB 61 comprising a pad 62 suitable to besoldered to a respective solder-ball of a device. Pad 62, which for theclarity of the explanation is substantially round, comprises a microvia63 according to some embodiments of the invention. A point 66 is theanticipated location of the crack initiation point on a perimeter 65 ofpad 62. A vector 60 initiated at point 66 is the projection onto pad 62of the anticipated crack propagation direction. The center 64 ofmicrovia 63 is located on vector 60 and is farther than the center 67 ofpad 62 from point 66. In addition, at least one point of perimeter 68 ofmicrovia 63 is located on perimeter 65 of pad 62. For the clarity of thedrawing, a dashed curve 69 is shown, which is an equidistance curve frompoint 66 containing the center 67 of pad 62.

A first instruction according to some embodiments of the invention, usedin the implementation of pad 62 and microvia 63 is to locate microvia 63inside pad 62 such that the center of microvia 63 is farther than thecenter of pad 62 from point 66.

A second instruction according to some embodiments of the invention,used in the implementation of pad 62 and microvia 63 is to locate center64 of microvia 63 on vector 60.

A third instruction according to some embodiments of the invention, usedin the implementation of pad 62 and microvia 63 is to locate microvia 63inside pad 62 such that at least one point of perimeter 68 of microvia63 is located on perimeter 65 of pad 62.

FIG. 8 shows an exemplary PCB 71 comprising a “footprint” 72. Footprint72 may be suitable to have installed thereon a device having a ball gridarray (BGA) package of sixteen solder balls arranged in an array of fourcolumns and four rows. Footprint 72 may therefore comprise sixteen pads73 suitable to be soldered to respective ones of the sixteensolder-balls of the device. Pads 73 have microvias 74 located therein.

Moreover, and although the scope of the invention is not limited in thisrespect, a projection 75 onto PCB 71 of the anticipated crackpropagation directions in solder balls of a device soldered to pads 73may point toward a common point 76, and an anticipated crack initiationpoint 77 of each solder ball may be located near the point of the padthat is the farthest from common point 76. Microvias 74 are located suchthat the center of microvias 74 are farther than the center of therespective pad 73 from common point 76.

FIG. 9 shows an exemplary PCB 80 in accordance with some embodiments ofthe invention. PCB 80 may comprise, for example, “footprints” 81, 82,83, 84, and 85 for respective devices. Mechanical features andadditional footprints of PCB 80 are not shown for clarity. Footprint 81may be suitable to have installed thereon a device having a ball gridarray (BGA) package of thirty solder balls arranged in an array of sixcolumns and five rows. Footprint 82 may therefore comprise thirty pads86 suitable to be soldered to respective ones of the thirty solder-ballsof the device.

PCB 80 may comprise traces (not shown) merging with some or all of pads86 according to embodiments of the present invention. In addition, PCB80 may comprise microvias located in some or all of pads 86 according toembodiments of the invention.

FIG. 10 shows an exemplary PCB 90 in accordance with some embodiments ofthe invention. PCB 90 may have installed thereon some devices, forexample, devices 93, 94, and 95, and may optionally have installedthereon a voltage monitor 92. PCB 90 may have installed on footprint 81a device 91 having a BGA package of thirty solder balls arranged in anarray of six columns and five rows. Mechanical features and additionalfootprints of PCB and additional devices installed on PCB 90 are notshown for clarity. As a non-limiting example, PCB 90 may be amotherboard.

FIG. 11 shows an exemplary apparatus 100 in accordance to someembodiments of the invention. Apparatus 100 may comprise PCB 90 of FIG.10, and may optionally comprise an audio input device 101. Anon-exhaustive list of examples for apparatus 100 includes a personalcomputer (PC), a notepad computer, a notebook computer, a laptopcomputer, a server computer, a pocket PC, a personal digital assistant(PDA), a personal information manager (PIM), a cellphone, a pager, amobile or non-mobile memory storage device, a hard disk drive (HDD), afloppy disk drive (FDD), a monitor, a projector, a digital video disc(DVD) player, a video compact disc (VCD) player, an MP3 player, a mobilemedia player, a calculator, a wireless local area network (LAN) accesspoint, a LAN router, and the like.

As a result of using instructions according to embodiments of theinvention during the implementation of a PCB, solder balls of a devicethat are soldered to pads of PCB implemented using these instructionsmay be more immune to crack initiation, and cracks may propagate moreslowly and/or may cause damage to the solder balls more slowly than ifthat PCB was implemented without these instructions. It will beappreciated that the instructions need not be satisfied for all the padsand all the traces to which the guidelines apply for the solder balls tobe more immune to crack initiation and for cracks to propagate moreslowly and/or to cause damage to the solder balls more slowly.

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those of ordinary skill in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention.

1. A printed circuit board comprising: pads suitable to be soldered torespective solder-balls of a device, said pads having microvias locatedtherein, where a center of a microvia of a pad is farther than a centerof said pad from a crack initiation point located on a perimeter of saidpad at a location where cracks in a solder-ball are anticipated to startafter said solder-ball is soldered to said pad.
 2. The printed circuitboard of claim 1, wherein at least one of said pads is a metal-definedpad.
 3. The printed circuit board of claim 1, wherein at least one ofsaid pads is a solder-mask-defined pad.
 4. The printed circuit board ofclaim 1, wherein at least one of said pads is substantially round. 5.The printed circuit board of claim 4, wherein a straight line joiningsaid crack initiation point and said center of said microvia is parallelto the projection onto the pad of the crack propagation direction forsaid solder-ball.
 6. The printed circuit board of claim 5, wherein atleast one point of the perimeter of said microvia is located on theperimeter of said pad.
 7. The printed circuit board of claim 1, whereinat least one point of the perimeter of said microvia is located on theperimeter of said pad.
 8. A printed circuit board having a deviceinstalled thereon, the printed circuit board comprising: pads solderedto respective solder-balls of said device, said pads having microviaslocated therein, where a center of a microvia of a pad is farther than acenter of said pad from a crack initiation point located on a perimeterof said pad at a location where cracks in a solder-ball are anticipatedto start after said solder-ball is soldered to said pad, wherein saidprinted circuit board has a voltage monitor installed thereon.
 9. Theprinted circuit board of claim 8, wherein at least one of said pads is ametal-defined pad.
 10. The printed circuit board of claim 8, whereinsaid printed circuit board is a motherboard.